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><A
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>35.12. User-defined Operators</A
></H1
><P
>   Every operator is <SPAN
CLASS="QUOTE"
>"syntactic sugar"</SPAN
> for a call to an
   underlying function that does the real work; so you must
   first create the underlying function before you can create
   the operator.  However, an operator is <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not merely</I
></SPAN
>
   syntactic sugar, because it carries additional information
   that helps the query planner optimize queries that use the
   operator.  The next section will be devoted to explaining
   that additional information.
  </P
><P
>   <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> supports left unary, right
   unary, and binary operators.  Operators can be
   overloaded;
   that is, the same operator name can be used for different operators
   that have different numbers and types of operands.  When a query is
   executed, the system determines the operator to call from the
   number and types of the provided operands.
  </P
><P
>   Here is an example of creating an operator for adding two complex
   numbers.  We assume we've already created the definition of type
   <TT
CLASS="TYPE"
>complex</TT
> (see <A
HREF="xtypes.html"
>Section 35.11</A
>).  First we need a
   function that does the work, then we can define the operator:

</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION complex_add(complex, complex)
    RETURNS complex
    AS '<TT
CLASS="REPLACEABLE"
><I
>filename</I
></TT
>', 'complex_add'
    LANGUAGE C IMMUTABLE STRICT;

CREATE OPERATOR + (
    leftarg = complex,
    rightarg = complex,
    procedure = complex_add,
    commutator = +
);</PRE
><P>
  </P
><P
>   Now we could execute a query like this:

</P><PRE
CLASS="SCREEN"
>SELECT (a + b) AS c FROM test_complex;

        c
-----------------
 (5.2,6.05)
 (133.42,144.95)</PRE
><P>
  </P
><P
>   We've shown how to create a binary operator here.  To create unary
   operators, just omit one of <TT
CLASS="LITERAL"
>leftarg</TT
> (for left unary) or
   <TT
CLASS="LITERAL"
>rightarg</TT
> (for right unary).  The <TT
CLASS="LITERAL"
>procedure</TT
>
   clause and the argument clauses are the only required items in
   <TT
CLASS="COMMAND"
>CREATE OPERATOR</TT
>.  The <TT
CLASS="LITERAL"
>commutator</TT
>
   clause shown in the example is an optional hint to the query
   optimizer.  Further details about <TT
CLASS="LITERAL"
>commutator</TT
> and other
   optimizer hints appear in the next section.
  </P
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